1. Field of the Invention
This invention relates to liquid crystal displays, particularly, active-matrix liquid crystal displays for use in electronic devices such as color image projectors, which operate with line inversion drives and column inversion drives.
2. Description of the Related Art
Conventionally, liquid crystal displays of the projection type (namely, liquid crystal display projectors) use active-matrix liquid crystal displays as their liquid crystal light valves, which act as light modulators. The active-matrix liquid crystal display contains an active-matrix substrate that fabricates signal lines electrodes, and switching elements for pixels, and its opposite substrate having common electrodes, wherein these substrates are arranged opposite to each other and are separated from each other with a prescribed gap via a seal material. A liquid crystal is held in such a gap between the substrates of the active-matrix liquid crystal display. A large number of xe2x80x98pixel electrodesxe2x80x99 are arranged on the prescribed display area of the display, and they are respectively encompassed by data lines and scan lines. Hence, the pixel electrodes are arranged in a matrix form on the screen of the display.
The recent mainstream technology for active-matrix liquid crystal displays is called a xe2x80x98Twisted Nematicxe2x80x99 (simply abbreviated in xe2x80x98TNxe2x80x99) mode. This is because the liquid crystal displays of the TN mode provide high brightness, high contrast, and relatively high-speed response, while they can be driven with relatively low voltages and are easy to control in gradations. That is, the liquid crystal displays of the TN mode provide various characteristics, which are essential for the existing displays, with a good balance. The TN mode employs the prescribed structure in which liquid-crystal molecules are twisted in their long-axis directions between the active-matrix substrate and its opposite substrate. Generally speaking, the twisted-nematic liquid crystal display (or xe2x80x98TNLCDxe2x80x99) uses a liquid crystal whose twisted-nematic molecules align on a helical axis in the absence of an electric field, twisting polarized light up to 90xc2x0.
An alignment direction for aligning liquid-crystal molecules is regulated by surface conditions of the substrates. That is, the liquid-crystal molecules cannot all always be aligned in the prescribed direction by simply aligning them in parallel to the screen surface of the liquid crystal display because they have a certain degree of freedom with respect to the alignment direction. One method for ensuring an alignment of liquid-crystal molecules in the specific direction is to physically control them in their long-axis directions by providing surfaces of the substrates with coating materials or channels directing an alignment in a specific direction. Specifically, the surface of the substrate is coated with a polyimide resin having a specific orientation to form an orientation film thereon. The orientation can be further enhanced by forming scratches extending in the specific direction on the surface of the orientation film. In addition, there are also provided several measures in orientation processes to provide a specific orientation to the orientation film formed on the surface of the substrate. For example, a so-called rubbing method is used to rub the orientation film with the cloth wound about a roll, or a slanted deposition (or slanted evaporation) method is used to deposit an inorganic material in a slanted direction to form an orientation film.
More specific descriptions will be made with respect to a typical example of the active-matrix liquid crystal display that uses thin-film transistors (namely xe2x80x98TFTxe2x80x99) as switching elements for pixels. That is, the active-matrix liquid crystal display is composed of an active-matrix substrate for fabricating scan lines, data lines, pixel electrodes and thin-film transistors, and its opposite substrate having common electrodes, wherein a liquid crystal layer is narrowly held in a gap between these substrates that are arranged opposite to each other and are separated from each other via a seal material.
On a front surface of the active-matrix substrate directly facing with the liquid crystal layer, a large number of data lines and scan lines are wired to intersect with each other in grid patterns in connection with thin-film transistors, so that each thin-film transistor is arranged in proximity to a point of intersection between each data line and each scan line. In addition, pixel electrodes are connected to the data lines and scan lines by means of the thin-film transistors respectively. One pixel is defined as a region that contains each one pixel electrode as well as its related data line, scan line, and thin-film transistor. Thus, the active-matrix liquid crystal display can display images of dots by activating respective pixels that are arranged in a matrix form.
Orientation films urging liquid-crystal molecules to prescribed orientation states in a non-power mode (or a power-off mode) where no voltage is applied between the substrates are respectively formed on surfaces of the active-matrix substrate and its opposite substrate sandwiching the liquid crystal layer. Conventionally, the orientation films are composed of orientational high molecular materials such as polyimide, so that organic orientation films whose surfaces are subjected to rubbing processes are widely used. In the rubbing process, the prescribed rubbing cloth is used to rub the surface of the film in a certain direction.
In the active-matrix substrate, regions forming data lines, scan lines, and thin-film transistors have a greater number of layers compared to regions forming pixel electrodes. That is, peripheral portions of pixels containing data lines, scan lines, and thin-film transistors are increased in height compared to center portions of pixels. This causes differences in height being formed between the peripheral portions and center portions of the pixels on the active-matrix substrate.
Recently, liquid crystal displays are manufactured with very fine structures by decreasing dimensions of pixels. In the rubbing process of the orientation film, the rubbing cloth does not make good contact with the differences and their neighboring areas on the active-matrix substrate. Thus, it is very difficult to perform the rubbing process completely on the entire surface area of the orientation film.
If the rubbing process is made incomplete with respect to boundary areas corresponding to neighboring areas of differences formed between peripheral portions and center portions of pixels, defectiveness may occur in these areas of the orientation film. In a non-power mode, liquid-crystal molecules will not be sufficiently regulated by the orientation film in proximity to the aforementioned boundary areas. This may cause orientation failures in which liquid-crystal molecules become unstable in orientation due to various factors. That is, a so-called xe2x80x98reverse tilt domainxe2x80x99 (i.e., a region in which liquid-crystal molecules have different directions in building up or tilting) is caused to occur at the boundary areas between the peripheral portions and center portions of the pixels. This may cause display failures such as leakage of light.
In the slanted deposition method, deposition may not be completely performed around peripheral portions of pixels because of shadows of the differences. For this reason, as the display is manufactured with a very fine structure, it may cause a noticeable increase for orientation-incomplete areas in which the orientation process was not performed completely with respect to the orientation film formed on the surface of the active-matrix substrate. In the orientation-incomplete areas substantially corresponding to the peripheral portions of pixels, liquid-crystal molecules are not sufficiently regulated in accordance with the prescribed orientation, so they become unstable in orientation due to various factors. In short, the substrate of the active-matrix liquid crystal display must withstand an orientation-failure state. This may cause so-called disinclination, i.e., a region that provides a difference of orientation directions of liquid-crystal molecules with respect to an area between the peripheral portion and center portion of the pixel. Therefore, a display failure such as leakage of light occurs at a boundary of the aforementioned region.
The aforementioned orientation failure can be solved by making the substrate surface flat or planar, thus allowing the orientation process to be completely performed on any portions of pixels. Specifically, it is possible to provide the following methods for the flattening or planarization of the substrate surface.
(i) Signal lines are embedded in channels that are formed on the surface of the substrate.
(ii) After wiring signals lines on the substrate, they are embedded in an insulation film having a high degree of planarization.
(iii) The insulation film is made flat or planar by using chemical mechanical polishing (abbreviated as xe2x80x98CMPxe2x80x99).
Conventionally, the active-matrix liquid crystal display has employed so-called called xe2x80x98frame inversion drivexe2x80x99, which provides polarity inversions for picture signals applied to the liquid crystal with respect to each of frames, in consideration of the lifetime (or service life) of the liquid crystal material. By using the frame inversion drive, it may be possible to increase the lifetime of the liquid crystal material. However, it causes flickering (fluctuations of lights or pictures) due to strokes between adjoining pixels on the screen, which may deteriorate the quality of the display. As solutions to flickering that may occur on the screen of the display, it is possible to use various types of display drive techniques called xe2x80x98column inversion drivexe2x80x99 and xe2x80x98line inversion drivexe2x80x99. That is, the column inversion drive provides polarity inversions for picture signals with respect to each of adjoining data lines, and the line inversion drive provides polarity inversions for picture signals with respect to each of adjoining scan lines.
Even though the orientation failure is solved by improving the planarization of the substrate, the aforementioned line inversion drive and column inversion drive may cause display failures due to the aforementioned disinclination at the peripheral portions of pixels. This is because in the line inversion drive and column inversion drive, picture signals having different polarities are supplied to adjoining pixels respectively so that a lateral electric field appears between the adjoining pixel electrodes of the active-matrix substrate in addition to a vertical electric field that appears between the pixel electrodes of the active-matrix substrate and the common electrodes of the opposite substrate, wherein the vertical electric field directly contributes to driving the liquid crystal. Under effects of the lateral electric field, liquid-crystal molecules may be disturbed in alignment.
It is an object of the invention to provide a liquid crystal display, employing line inversion drive and/or column inversion drive, which can reduce display failures due to disinclination caused by a lateral electric field that appears between adjoining pixel electrodes on a substrate.
An active-matrix liquid crystal display of this invention is basically composed of an active matrix substrate fabricating signal lines and pixel electrodes, a liquid crystal, and a opposite substrate having a common electrode. The liquid crystal is held between the active matrix substrate and opposite substrate, which are arranged opposite to each other. On the active matrix substrate, there are arranged a first group of pixel electrodes that are aligned in a prescribed direction and are supplied with picture signals of a first polarity, and a second group of pixel electrodes that are aligned to adjoin with the first group of pixel electrodes respectively and are supplied with picture signals of a second polarity. Liquid-crystal molecules lying in proximity to the active matrix substrate are initially oriented in a first orientation direction (a) in a non-power mode in such a way that their long-axis directions are parallelized with alignment directions of the first and second groups of pixel electrodes. Alternatively, they are oriented in such a way that their long-axis directions cross with the alignment directions of the first and second groups of pixel electrodes. In this case, they are twisted in such a way that their long-axis directions extend from the active matrix substrate to the opposite substrate and lie across the first and second groups of pixel electrodes on the active matrix substrate in plan view. In addition, liquid-crystal molecules lying in proximity to the opposite substrate are initially oriented in a second orientation direction (Rb) that rectangularly crosses the first oration direction.
In the active-matrix liquid crystal display of this invention, an inorganic material such as a silicon oxide is subjected to slanted deposition to form an inorganic orientation film on the surface of the active matrix substrate, while an organic orientation film is formed on the surface of the opposite substrate that is composed of orientational high molecules of a polyimide. The slanted deposition is performed one time to form one type of pillar structure in which pillars are arranged and are slanted in a specific direction on the active matrix substrate. Alternatively, the slanted deposition is performed multiple times to provide mixtures of pillar structures that are slanted in different directions. Thus, it is possible to form an orientation film without using an organic orientation film requiring the rubbing process that may result in incomplete orientation even though the active matrix substrate have many differences in height. Therefore, the rubbing process is performed with respect to the organic orientation film of the opposite substrate. The orientation films apply different pre-tilt angles to liquid-crystal molecules in relation to the active matrix substrate and opposite substrate respectively. That is, the first pre-tilt angle, which preferably ranges from 3xc2x0 to 30xc2x0, imparted to liquid-crystal molecules lying in proximity to the active matrix substrate becomes larger than the second pre-tilt angle imparted to liquid-crystal molecules lying in proximity to the opposite substrate.
The aforementioned orientations reliably reduce influences of lateral electric fields, which occur at a power-on mode in response to line inversion drive to cause unwanted motions for liquid-crystal molecules, in particular, liquid-crystal molecules lying between adjoining pixel electrodes electrified at different potentials respectively in proximity to the active matrix substrate. Thus it is possible to reduce the occurrence of disinclination due to lateral electric fields, thus reducing display failures.